1. Field of the Invention
This invention relates to a servo apparatus for controlling the revolution of a motor so as to prevent distortion of a reproduced image resulting from irregularity of rotation in an apparatus for reproducing picture signals recorded on a magnetic disk or a magneto-optical disk.
2. Related Background Art
The standardization of video floppies for endlessly recording picture signals corresponding to one field or one frame on one track or two tracks of a magnetic disk has been made, and an electronic still camera or the like using a video floppy as a recording medium has been announced.
In an apparatus for reproducing picture signals from a magnetic disk and displaying the images on a Braun tube or printing the images on paper, it is necessary to control the revolution of a disk driving motor by the use of the vertical synchronizing signal and horizontal synchronizing signal of a reproduced picture signal in order to eliminate distortion of the reproduced image resulting from the irregularity of rotation of the disk during recording.
As prior art, there is to be found reproduction servo control of a VTR which handles the picture signal, and in which, for example, a vertical synchronizing signal (hereinafter referred to as "VS signal") and a horizontal synchronizing signal (hereinafter referred to as "HS signal") separated from the reproduced picture signal are phase-compared with a reference signal put out from a synchronizing signal generator to thereby servo-control the driving motor.
In the recording-reproducing apparatus of electronic still camera, however, picture signals corresponding to one field or picture signals corresponding to one frame are successively recorded in an endless form on one track or two tracks of a disk. During reproduction, the picture signals corresponding to one field are repetitively reproduced or the two tracks are alternately reproduced to thereby repetitively reproduce the picture signals corresponding to one frame. Thus, during reproduction, a VS signal is produced for each one full rotation of the disk irrespective of the presence of irregularity of rotation during recording. Even if servo control is applied for the phase comparison between the reproduced VS signal and the reference VS signal, the disk is only rotated at 60 HZ, i.e., 3600 r.p.m., and distortion of the reproduced image caused by the irregularity of rotation during recording which has occurred as a fluctuation of the rotational speed during one full rotation cannot be corrected.
Further, when servo control is applied with the reproduced HS signal and the reference HS signal being phase-compared, picture signals corresponding to one field or one frame are repetitively reproduced in the reproduction of the electronic still camera and therefore, the frequency of the irregularity of rotation of the reproducing signal is as high as 60 Hz for one field or 30 Hz for one frame, in the case of the NTSC system. As compared with the reproduction servo effected in the conventional VTR or the like, the frequency of the irregularity of rotation becomes very high and, when considered from the transfer function of the motor driving system, the phase delay at a high frequency is great, and distortion of the reproduced image caused by the irregularity of rotation during recording cannot be corrected by the reproduction servo heretofore effected in the conventional VTR or the like.
Also, selection of a high response frequency of the servo system has posed a problem that rotation of the disk is liable, to be disturbed when noise mixes with the reproduced HS signal.